This invention is directed to a thin film format of magnetic medium which is capable of high density magnetic recording, particularly longitudinal recording. The invention is also directed to having such a medium capable of use in magnetic sensing in a magnetoresistive ("MR") head.
Current art indicates that conventional magnetic recording and sensing mediums, such as cobalt alloy media, for instance, CoPtCr (Cobalt, Platinum, Chrome), are unable to achieve recording densities above 2 to 5 Gbit/in.sup.2. An historic trend has been to require reduced magnetic areal moment ("Mrt") and increased coercivity ("Hc") for recording at higher densities. This trend has been extrapolated to theoretical models for 10 Gbit/in.sup.2 recording densities. E. S. Murdock, IEEE Trans. Mag. 28, 3078, 1992.
High density recording media also need to consist of exchange decoupled particles. Smaller particles are required at higher densities for reasons of reduced noise, and thereby the ability to obtain a higher signal to noise ratio ("SNR"). Thus, high density recording media requires a reduced Mrt and a reduced particle size. This has been theoretically estimated at 8 to 10 nm for 10 Gbit/in.sup.2. E. S. Murdock, IEEE Trans. Mag. 28, 3078, 1992.
Coercivity, Magnetic Moment and Particle Size
Hc, Mrt, and particle size for a 1 Gbit/in.sup.2 demonstration and for a theoretical model of a 10 Gbit/in.sup.2 requirement are shown in Table I. E. S. Murdock, IEEE Trans. Mag. 28, 3078, 1992; T. Yogi, C. Tsang, T. Nguyan, K. Ju, G. Gorman, and G. Castillo, IEEE Trans. Mag., 26, 2271, 1990.
TABLE I ______________________________________ 10 Gbit 10 Gbit 1 Gbit Case 1 Case 3 Demo (Theoretical Model) (Theoretical Model) ______________________________________ tpi (track density) 6,350 11,000 25,000 kbpi (linear density) 158 909 400 HC (Oe) 1800 4500 2500 Mrt (memu/cm.sup.2) 0.7 0.35 0.5 Grain size (nm) 15-20 8-10 8-10 ______________________________________
It is not expected that conventional cobalt alloy systems can meet the Hc, Mrt, and particle sizes shown for high density recording in mediums and ranges of greater than 1 Gbit/in.sup.2 and particularly approximating about 10 Gbit/in.sup.2. In fact, scaling of the 1 Gbit/in.sup.2 demonstration to 10 Gbit/in.sup.2 suggests an even smaller particle size than that shown, namely as small as 5 to 7 nm, would be needed.
Anisotropy and Superparamagnetism
For all magnetic material systems Hc eventually drops as particle size is reduced due to thermally assisted switching, a phenomena described as superparamagnetism. B. D. Cullity, "Introduction to Magnetic Materials", Addison-Wesley Publishing Co., Reading MA, 1972.
It has not been possible in the prior art to attain all hexagonal phase cobalt alloy systems with a media Hc/Mrt ratio needed to permit for 10 Gbit/in.sup.2 densities. I. L. Sanders, T. Yogi, J. K. Howard, S. E. Lambert, G. L. Gorman, and C. Hwang, IEEE Trans. Mag., 25, 3869, 1989. Hc falls off as Mrt is reduced in the 0.4 to 0.6 memu/cm.sup.2 range or as grain size or particle size, if exchange decoupled, is reduced.
To achieve a smaller particle size without superparamagnetic effects, a higher crystalline magnetic anisotropy is required. B. D. Cullity, "Introduction to Magnetic Materials", Addison-Wesley Publishing Co., Reading MA, 1972. Table II compares the crystalline anisotropy expressed as the intrinsic coercivity field for coherent rotation of an isolated spherical particle and superparamagnetic particle size limit calculated for a spherical isolated particle for several different magnetic materials.
TABLE II ______________________________________ Hci = 2K/Ms Grain size (Oe) minimum (nm) ______________________________________ Co-alloy* 4,000 15.0 Barium Ferrite 17,000 9.7 CoPt 80,000 5.3 SmCo.sub.5 200,000 3.4 ______________________________________ *Co--19% Cr, T. Weilinga et al, IEEE Trans. Mag., V 18, p 1107, 1982.
The conventional hexagonal phase cobalt alloys are represented by a Co - 19 at. % Cr alloy. Corresponding values for a CoPtCr, CoPtCrB, or CoCrTa alloy can be assumed similar.
Additional alloying elements may increase anisotropy which, acting alone, would reduce the limiting superparamagnetic particle size. This improvement is however mitigated by the moment dilution that occurs with the addition of the alloying elements. Experimental data of hexagonal cobalt alloys do not show suitable Hc/Mrt and particle size.
Table II identifies several alternative materials systems with a theoretical potential to achieve 10 Gbit/in.sup.2 requirements.
The L1o Ordered Phase of Magnetic Material
The L1o phase has a tetragonal crystal structure and is related to its disordered FCC solid solution. With a CoPt alloy, for example, there is a layering of the Co and Pt atoms, in alternate planes, along the FCC [001] direction. This direction then becomes the c-axis of the tetragonal L1o.
An L1o ordered phase medium is, for example CoPt, with a compositional range of 45 to 60 atomic percent cobalt. Such a material is known as a high magnetic anisotropy (Hc=4,000 to 5,000 Oe) permanent magnet material. D. J. Craik, Platinum Met. Rev., 16, 129, 1972; A. S. Darling, Platinum Met. Rev. 17, 95, 1963.
The c-axis of the L1o is similar to the c-axis of hexagonal cobalt in that both are the easy axis of magnetization for the crystal. Thus, while the disordered FCC solid solution of Co and Pt has cubic symmetry and low magnetic anisotropy, the ordered L1o phase has uniaxial anisotropy similar to, but greater in magnitude, than hexagonal cobalt.
Thin films of this L1o material have been prepared by previous workers. V. Tutovan, V. Georgescu, and H. Chiriac, Thin Solid Films, 103, 253, 1983; J. A. Aboaf, S. R. Herd, E. Klockholm, IEEE Trans. Mag., 19, 1514, 1983; A. Tsokatos, G. C. Hadjipanayis, C. P. Swann, and S.I. Shah, page 701 in "Science and Technology of Nano-structured Magnetic Materials", Plenum Press, New York N.Y., 1991.
A very high Hc has been reported (max testing field 5,000 Oe) at room temperature. J. A. Aboaf, S. R. Herd, E. Klockholm, IEEE Trans. Mag., 19, 1514, 1983. Films with such materials are still relatively too thick, namely greater than 500 .ANG., for currently required applications. U.S. Pat. No. 4,438,066 (Aboaf). An Hc of about 10,000 Oe at 10.degree. K has been reported by another group. A. Tsokatos, G. C. Hadjipanayis, C. P. Swann, and S. I. Shah, page 701 in "Science and Technology of Nano-structured Magnetic Materials", Plenum Press, New York N.Y., 1991.
Thin epitaxial films in the range of 100 .ANG. to 500 .ANG. with a coercivity of 4000 to 8000 have been reported. Such films are epitaxial or monocrystalline and thus are not suitable for use as a magnetic recording films, particularly in the longitudinal mode. V. G. Pyn'Ko, L. V. Zhivayeva, N. A. Ekonomov, A. S. Komalov, N. N. Vevtikhiyev and A. R. Krebs, Fiz. metal metalloved., 45, No. 4, 879-881, 1978.
A. Tsoukatos, G. C. Hadjipanayis, Y. J. Zhang, M. Waite and C. P. Swaim, Met. Res. Soc. Symp. Proc. Vol. 232. 1991 Materials Research Society has reported that the magnetic hysterisis of atomic CoPt films is such that as the film decreases in thickness the coercivity reduces.
None of the prior art with regard to CoPt is directed to thin film material suitable for high density recording media. The teachings suggest that as the material becomes thinner the coercivity decreases below a level suitable for high density recording. In particular, as the thickness of polycrystalline L1o material becomes less than 500 .ANG. the coercivity decreases below acceptable levels. Despite the work done on such films, it has not been possible to provide sufficiently thin films suitable for effective use as a high density recording media.
There is a need to provide a sufficiently thin film magnetic medium capable of achieving high recording densities. Such material should ideally have a coercivity sufficiently high to operate with hardware so as to effectively permit recording and sensing of the magnetic signal. The medium should also ideally have a high anisotropy.
More particularly there is a need to provide a thin film medium, preferably less than a few hundred Angstrom, capable of recording in a longitudinal direction and having recording densities at least as high as about 10 Gbit/in.sup.2.